The present invention relates generally to surface modified stainless steel with increased resistance to high temperatures. In particular, it relates to FeCrAl alloys that are modified by the application of a Ca-containing compound on their surface.
It is known art to use FeCrAl alloys for applications with high requirements for heat resistance, such as for example purification of automobile exhaust gases by using catalytic converters made of metallic substrates or electrical resistance heating applications. Aluminum is added to the alloy to form an alumina layer on the surface of the alloy after heat treating the alloy. This alumina is considered to be one of the most stable oxides having low oxidation rate at high temperatures. FeCrAl-alloys, forming aluminum oxide at exposure to high temperatures, e. g. above 1000xc2x0 C., especially in thinner dimensions, for instance 50 xcexcm foils for use in catalytic converters in the automobile industry, have a limited lifetime. This is due to breakaway oxidation, oxidation of Fe and Cr and that the matrix is depleted of Al after aluminum oxide formation after certain periods of time of use in cycles of high temperatures. Common conventional methods of increasing lifetime are the following:
alloying with Rare Earth Metals (REM) and/or Yttrium in order to increase the oxidation resistance of the FeCrAl alloy by supporting the forming of an aluminum oxide layer on the surface of the alloy.
increasing the aluminum content, or the contents of other elements with high oxygen affinity, in the matrix, which often leads to production difficulties such as embrittlement during rolling
cladding the material with aluminum foils.
These methods have to rely on time consuming diffusion controlled processes. It is therefore an object of the present invention to provide a new approach how to increase the resistance to corrosion at high temperature, especially at cyclic thermal stress, and thereby increase the lifetime of said type of alloy.
By applying a continues uniform layer of a Ca-containing compound on the surface of the FeCrAl alloy before annealing, a mixed oxide of Al and Ca is formed during the heat treatment. This treatment gives the advantage of influencing, i e hindering, the aluminum oxide formation and nucleation already during the beginning of exposure to high temperature, which increases the lifetime more effectively than other methods, e g alloying or cladding. The surface has a more compact and homogenous oxide layer with less pores, dislocations and cavities than the hitherto known alumina layers formed on FeCrAl-alloys after heat treatment. The surface layer acts as barrier for aluminum ions and oxygen to diffuse through the alloy/oxide boundary and the oxidation resistance and lifetime of the alloy are therefore significantly improved. It is believed that the Ca-layer on the surface of the alloy tightens the surface in a way that the alumina depletion of the alloy is drastically reduced. Ca also favors the selective oxidation of Al, which improves the oxidation resistance at elevated temperatures and the lifetime of the alloy.